The ability to produce oil and/or gas from a subterranean well may be improved by injecting chemicals/additives into the well. An injection pump can inject various additives for different applications, such as a foaming agent to increase gas production, a corrosion/scale inhibitor to protect tubing from damage/build-up, and/or methanol to prevent gas from freezing in a production line. Depending on the specification of the well and/or the application, the pump may have to inject a different amount of additive for each well or each type of additive. Also, depending on the application, the additive may need to be pumped into the well in one batch per day or in multiple batches per day. For example, if an additive is injected into a well in one batch per day this is referred to as one cycle per day. Likewise, if an additive is injected into a well in four batches at four times that are equally spaced over a day, this is referred to as four cycles per day. In some applications, it is desirable to inject small batches of additives at short intervals throughout a day for production purposes. In an application where additives are injected once per minute for an entire day, there are 1440 cycles per day. Cycles per day may also be referred to as pump cycles or injection cycles.
In theory, an injection pump runs at a constant speed so that a controller need only operate the pump for fixed temporal durations intermittently for the desired number of cycles to inject the desired amount of chemical into the well/pipeline. However, in practice, the pump injection rate varies with each well site due to the wellhead conditions such as: (a) the point where the additives are injected into the well, for example some additives are injected at the wellhead at ground level and some additives are injected down into the borehole of the well itself; (b) the wellhead pressure at the point of injection; (c) the size of injection lines between the chemical tank and the point of injection; (d) the type and number of fittings in injection lines between the additive tank and the point of injection; (e) the length of the injection lines between the additive tank and the point of injection; and (f) the viscosity of the additive, which may vary based on temperature. Further, in remote applications, pumps are often run by DC sources (e.g., solar cells or batteries) and variation in the voltage or current of the power source may affect the speed of the pump. Finally, pumps and associated components (e.g., check valves, etc.) wear over time resulting in changing operating parameters. Any of these factors may affect the pump injection rate of a pump.
To account for variations in pump injection rates, previous systems have required that operators run an injection rate test at the well to determine how fast the pump injects additives into a specific well. This information is utilized to determine what operating duration (e.g., pump on time) is required for the injection pump to inject a desired volume of additive into the well for each injection cycle. However, such an approach is only valid if the system remains static. This is, if there are changes in the well parameters (e.g., well head pressure) and/or the pump operation (e.g., variation in power supply), the operating duration required to inject a desired amount of additive changes.
Another approach is shown in the patent to Burns, Sr. et al. (U.S. Pat. No. 7,277,778) (Burns) which discloses a chemical injection pump system for wells with a controller taking commands from a local operator's control panel and from remote operator's control panel. The operator selects a specific injection pump type from the data files within the controller where the pump type selected in the controller is a very specific chemical injection pump type having a very specific pumping capacity and this specific injection pump type is used by the controller to compute the number of strokes required to dispense a desired volume of additive into the well. The controller in Burns is connected to a first sensor and a second sensor. The first sensor is for sensing a deactivated state of the pump. The second sensor is for sensing an activated state of the pump, to dispense a pre-determined quantity of chemical and to verify that the pump has actually operated. The controller assumes that all pumps of same type inject at the same rate without consideration of the wellhead conditions which vary significantly from well to well. Further the system typically requires expensive specialized pumps.
It is desirable to tightly control per cycle injection volumes for a number of reasons. In near continuous injection applications (e.g. 1440 cycles per day; once per minute) the repeated injection of small volumes of additives may significantly increase production of a well. Thus, it is often desirable to inject at least a minimum target volume of additives during each injection cycle; under injection can effect production. In contrast, over injection of additives often produces no production benefit and can be a significant operating expense. For instance, for a producer operating a thousand wells each requiring 6 L of additive per day, with an exemplary cost of $10/L, an over injection rate of 50% (9 L per day, per well) results in $360,000 in annual excess additive expenses.
Accordingly, it would be desirable to provide a chemical injection controller that controls pump operation based on actual volumes injected by the pump. Such a controller may inject a desired volume of additives irrespective of changes in well/pipeline pressures or conditions and/or variations in pump operation. Finally, it would be desirable for such a controller to minimize over injection while maintaining a desired per cycle injection volume.